The present invention relates to a glass ceramic for use especially as a relatively thin substrate in the most various applications. These include, inter alia, the use as thin-film semiconductor substrates, especially as thin film silicon, and as substrate for display applications (thin film transistor TFT display substrate, back plate, transparent front plate, etc.), as solar cells and the like, and as hard disk substrate.
There have been known in the art alkali-free glass ceramics of the basic glass system MgO—Al2O3—SiO2 (MAS System) with diverse admixtures in the form of glass-forming elements, network modifiers, intermediate oxides, nucleation agents and fluxing agents, which are obtained by tempering corresponding starting glass materials, whereby they are converted to materials with homogeneously distributed crystallites in a residual glass phase.
In this connection, there have been especially known numerous MAS glass ceramics for use as substrates for hard disk memories. Applications to be mentioned in this connection include, for example, U.S. Pat. No. 6,583,077; U.S. Pat. No. 5,968,857; U.S. Pat. No. 6,569,791; U.S. Pat. No. 6,458,730; U.S. Pat. No. 6,458,729; U.S. Pat. No 6,495,480; U.S. Pat. No. 5,491,116; EP 1 067 10; EP 0 941 973; EP 0 939 396 and EP 0 939 395.
The respective glass ceramic substrates partly contain admixtures of TiO2, P2O5, ZrO2 as nucleation agents for crystallization in given quantities. Further, alkaline earth oxides may be admixed in given quantities. The refining agents used in this connection are, as a rule, Sb2O3, As2O3 or SnO2. In some instances, additions of boron oxide (B2O3) of up to 4 wt. % are also envisaged in this connection (compare EP-A-0 939 396 and EP-A-0 939 395). Quite a number of glass ceramic materials, with given composition ranges for that application, have been known also from EP 0 941 973. These glass ceramics are, however, free of boron oxide. The refining agent used is again CeO2, As2O3 or Sb2O3.
Further, there must be mentioned different MAS glass ceramics used in connection with optical display means, such as LCDs (compare for example U.S. Pat. No. 6,197,429). As is generally known, the necessary transparency in the visual range of the electromagnetic spectrum (380 to 780 nm) can be achieved in this case by suitable control of the ceramization process so that the crystallite sizes are significantly smaller than the wavelengths of the respective light (i.e. smaller than approximately 300 nm) as in that case no diffusion of light will occur on the crystallites (compare Beall and Pinckney: “Nanophase Glass-Ceramics” J.Am.Ceram.Soc., 82(1) [1999]5-16. ) The crystallite sizes are kept reasonably small by corresponding purposeful nucleation and controlled crystal growth.
According to the state of the art, crystal phases normally separated in glass ceramics of the MAS system are cordierite, spinel, sapphirine, mullite, enstatite or forsterite (or corresponding mixed crystals in a siliceous residual glass phase. It is understood that the composition of the glass ceramics described in the prior art vary over a broad range—in correspondence with the composition of the separated crystal phases and residual glass phases.
In order to achieve a uniform distribution of crystallite sizes in the separated crystal phases an optimally high number and optimally homogeneous distribution of nuclei is required, which are produced, according to the prior art, by thermally induced separation in the glass, followed by the formation of nanocrystallites. One introduces for this purpose into the glass so-called nucleation oxides (TiO2, ZrO2) either individually or in given mixing ratios. When heating up the glass, usually to temperatures above Tg, one starts out in the case of TiO2 in the MAS system from the formation of Mg titanate nanocrystallites as nuclei for other crystal phases (compare Golubkov et. al.: “On the phase separation and crystallization of glasses in the MgO—Al2O3—SiO2—TiO2 system”, Glass Phys. Chem., 29/3 [2003] 254-266).
When the glass ceramics are to be used as self-supporting substrates, main importance is placed on properties such as high breaking strength and a high modulus of elasticity (specific stiffness: E/ρ). These properties can be influenced depending on the composition of the residual glass phase and the separated crystal phases and on the crystal phase proportions. In most of the cases, it is desired to achieve a specific stiffness of 30 to 50 MJ/kg. the MAS system glass ceramics heretofore known in the art usually have a thermal expansion coefficient of approximately 2 to 6×10−6/K in the temperature range of between 30 and 300° C.
MAS glass ceramics are presently in a development stage also as substrate for thin film silicon as base for active devices in integrated circuits, such as diodes or thin film transistors (TFT). By the use of substrate-based thin film silicon it is possible to produce components for flat screen displays (such as LCDs) and solar cells for generation of electric current and the like.
At present, it is predominantly amorphous thin film silicon (a-Si) that is applied on substrates. The temperatures required for the processes of depositing the amorphous thin film silicon on the substrates usually lie in the range of approximately 450° Celsius. The use of polycrystalline thin film silicon (poly-Si) in the respective components would present some decisive advantages compared with a-Si components, the poly-Si having a clearly higher electron mobility. For example, resolution and reaction speed of an LCD could be significantly increased. Further, this opens up new ways of on-board integration of additional integrated circuits installed, in the case of a-Si devices, on the edge of the LCDs, for example in the form of extra chips. Poly-Si is obtained in the art by recrystallization of a-Si on the substrate. In principle, this process is realized by heating the Si layer up to temperatures at which the a-Si crystallizes. One differentiates in this connection between low temperature poly-SI, which is obtained by heating the Si layer up locally to 600° Celsius, and high temperature poly-Si formed at process temperatures of approximately proximately 900° Celsius. For purposes of producing such poly-Si articles, either the entire component is heated up to the respective temperature (HT poly-Si), or else the desired temperatures are brought about locally by moving an excimer laser across the surface (the surface layer) in a corresponding raster pattern. The poly-Si of the components produced by the last-mentioned process frequently is not uniform. Such components may, for example, show so-called “pinpoint defects”, which are undesirable. In order to reach the same degree of integration as in the case of high-temperature poly-Si, low-temperature poly-Si components must be processed for a long time, usually over more than 20 hours.
Given the fact that in order to achieve a high degree of integration of the transistors a plurality of photolithographic processes are required, the poly-Si component must of course stand the temperature of the recrystallization cycles without essential changes in geometry (shrinkage) so that misalignments among the superimposed layers and with contact points, if any, are avoided. Usually, the shrinkage tolerance is a fraction only of the lateral extension of the smallest circuit unit implemented; compared with the entire substrate it is frequently limited to 50 ppm. In order to avoid stresses between the substrate and the Si layer, the coefficient of thermal expansion of the two materials must be adapted, or must be equal. The poly-Si components, heretofore only suited for use in poly-Si components, consist of amorphous SiO2 (silica glass) and are complex and costly to produce. In addition, the difference between the coefficients of thermal expansion of poly-Si and silica glass is approximately Δα30-300≈3.2×10−7/K.
In these cases, MAS glass ceramics designed specifically to the desired properties would provide on the one hand technical improvements and, on the other hand, considerable cost savings.
In this regard, for example U.S. Pat. Nos. 5,968,857 and 6,197,429 have become known.
None of the afore-mentioned glass ceramics considers, for example, the specific demands to be met in producing such glass ceramics in the form of plates with the smallest possible thickness.
It is, therefore, a first object of the present invention to provide a glass ceramic which is suited as a substrate for optical and electronic components.
It is a second object of the invention to disclose a glass ceramic which can be produced with high precision and high homogeneity even in small thicknesses, and this even in the case of large surface areas.
It is a third object of the invention to disclose a glass ceramic having a high specific modulus of elasticity and advantageous mechanical properties.
It is a fourth object of the invention to disclose a glass ceramic having a coefficient of thermal expansion that is controllable in a suitable way to allow the material to be used as a substrate for polysilicon.
It is a fifth object of the invention to disclose a glass ceramic that is transparent, if desired for certain applications.
Finally, a suitable production method for such glass ceramics is to be provided.